An apparent surge in cases of Barmah Forest virus last year was down to a batch of faulty tests, which gave hundreds of false positives.

A faulty batch of tests was behind an apparently massive spike in cases of the mosquito-borne Barmah Forest virus last year.

For the year to September 2 in 2013, there were 913 reported cases of the virus in Western Australia, up from 108 for the previous corresponding period and compared to 43 for the current year to date.

WA Health Department state medical entomologist Peter Neville said the faulty tests were used throughout Australia last year and gave hundreds of false positives, skewing figures nation-wide.

Dr Neville said the cases may have instead been Ross River virus, which has the same symptoms.

That still meant there had been a surge in arboviruses but that could be put down to climate patterns.

Every three to four years, there is an outbreak in mosquito-borne viruses.

"When we have very wet years, obviously rain can lead to pooling and mosquito breeding. We quite often see more cases of Ross River," Dr Neville told AAP.

"Also, it depends on tides. If we're having an increase in the frequency of tides and also the height of tides, that can lead to flooding of low-lying lands, which leads to bigger populations.

"This is all related to the Southern Oscillation Index and El Nino versus La Nina weather conditions."

Dr Neville said Ross River and Barmah Forest viruses afflicted marsupials, particularly kangaroos and possums, where they then "amplify".

"When we have ... lots of rainfall and lots of grass grows, we quite often see booms in marsupial populations and of course when they're born, they've got no immunity to these diseases, so we can see a boom in Ross River and Barmah Forest."

Dr Neville said the Department of Health and local councils were doing a lot to combat arboviruses including testing riverbeds for larvae, killing mosquitoes with chemical treatments and creating drainage in low-lying areas.

It was also important for members of the public to prevent pot plant bases, pet water bowls and gutters from becoming mosquito breeding grounds, he said.

Copyright 2014 AAP